Ultrasonic defect inspection assemblies and methods are known for inspecting objects such as gas turbine engine components and the like for defects such as cracks, inclusions and porosity.
Prior ultrasonic inspection methods comprise using an ultrasonic wave generator (transducer) and one or more ultrasonic wave detectors separated by a known difference to measure the time-of-flight of propagating ultrasonic waves along known ray-paths to determine the velocity of the ultrasonic waves and therefore the local density and other properties of the material. In some materials, known as “isotropic” materials, the wave velocity is substantially constant in all directions. However, some materials, such as single crystal or directionally solidified metals or alloys, exhibit anisotropic properties due for example to the orientation of the crystal, such that the wave velocity varies according to direction of wave propagation. In order to reliably inspect objects comprising anisotropic materials, the orientation of the crystal must therefore be determined prior to defect inspection.
Ultrasonic inspection methods can be used to determine crystal orientation and therefore wave propagation velocity anisotropy by propagating ultrasonic waves along known ray paths in a number of directions. From these velocity measurements, a prediction of the crystallographic orientation can be made. A number of different ultrasonic methods have been devised. A first method, as described for example in U.S. Pat. No. 4,106,327, uses a transducer located on one surface of the object and a detector located on an opposite surface to measure the velocity of bulk ultrasonic waves through the object. However, the method described in U.S. Pat. No. 4,106,327 requires access to both sides of the object. A second method, such as that described in U.S. Pat. No. 5,955,671 utilizes reflectors in the object having known locations and reflective properties to reflect the wave generated by the transducer back to a detector located on the same face as the transducer (known as “pulse-echo” or “pitch-catch”). This is not always possible, since such known reflectors may not be present, which is the case for in situ inspections of gas turbine engine components for example.
A third orientation method has been suggested which comprises using ultrasonic surface-waves. Such a method would have the advantage of only requiring access to a single face of the material to perform the measurement without requiring the presence of a known reflector within the object. However, such methods have not previously been successful using conventional detector arrays, since the signals received by the detectors have been found to have a low signal to noise ratio. As such, the wave velocity of the surface-waves can often not be measured, and hence the orientation cannot be predicted accurately.
The present invention provides an ultrasonic inspection assembly which seeks to overcome these disadvantages.